Prior to the present invention, it has been known that aniline may be condensed in the presence of certain catalysts to make diphenylamine and ammonia. See, for example, U.S. Pat. No. 2,098,039. It has been generally thought, however, that the reaction must take place either in the liquid phase or at temperatures well over 400.degree. C.
The liquid state is a required condition for several previously patented processes, such as the one recited in U.S. Pat. No. 2,645,662. Other prior art references employ somewhat specific catalysts, as U.S. Pat. Nos. 2,656,389; 4,100,195 and 2,120,968.
In the relatively simple reaction of aniline with itself in the presence of a catalyst to form diphenylamine, ammonia is evolved. The equilibrium concentrations of diphenylamine as a function of the starting molar ratio of ammonia to aniline, for the aniline condensation reaction, are as follows, based on Vriens & Hill, I&EC, November, 1952.
TABLE I ______________________________________ Equilibrium Wt. % DPA NH.sub.3 /Aniline Molar Ratio in Aniline ______________________________________ 0 84 0.5 62 1.0 48.5 2.0 33.5 3.0 25.5 ______________________________________
From this data, yields of diphenylamine from the reaction would, not surprisingly, appear to be suppressed by the presence of ammonia in the reaction zone.
Many of the workers in the prior art have made provisions in one way or another to remove the ammonia from the reaction site as soon as it is formed, usually to avoid suppressing the reaction. See, for example, column 2, line 5 of U.S. Pat. No. 2,820,829 and column 6, line 13 of U.S. Pat. No. 2,514,430. In U.S. Pat. No. 3,071,619, a liquid phase reaction is maintained with the bleeding off of ammonia from the reaction system--see column 2, lines 31-35.
The reader perhaps may be most interested in U.S. Pat. Nos. 2,968,676; 3,118,944 and 3,944,613. In the first-mentioned of these, a process is proposed in which an alumina catalyst is impregnated with aluminum fluoride and the reaction takes place in the vapor phase. In the second, a variety of conditions and functions are explored for the control of the vapor phase reaction, emphasizing the use of relatively high feed rates. In the third patent, an amorphous synthetic silica-alumina catalyst is employed for a liquid-phase reaction.
None of the above references employs ammonia as a carrier gas for the aniline into the reaction zone and/or to the catalyst, as we propose. While U.S. Pat. No. 2,082,815 suggests that the diphenylamine product can be used as a heat-exchange medium, this suggestion also falls short of my invention, which postulates that, among other purposes, the ammonia produced, if recycled to serve as a carrier for the aniline reactant, can serve as a heat sink for the reaction. The temperature range of our invention is especially noteworthy in view of U.S. Pat. No. 2,938,055, which carefully conserves energy while maintaining a high reaction temperature.
None of the above references utilizes a catalyst of the type we use under the temperature and phase conditions we use, the combination of which has provided exceptional selectivity and conversion rates. Also, most of the prior art for vapor phase reaction teaches that frequent catalyst regeneration is necessary to maintain catalyst activity. Our elimination of this need is commercially very significant.